Intra prediction mode mapping method and device using the method

ABSTRACT

The present invention relates to an intra prediction mode mapping method and a device using the method. The intra prediction mode includes: decoding flag information providing information regarding whether an intra prediction mode of a plurality of candidate intra prediction modes for the current block is the same as the intra prediction mode for the current block, and decoding a syntax component including information regarding the intra prediction mode for the current block in order to induce the intra prediction mode for the current block if the intra prediction mode from among the plurality of candidate intra prediction modes for the current block is not the same as the intra prediction mode for the current block. Thus, it is possible to increase the efficiency with which are images are decoded.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 15/958,452 filedApr. 20, 2018, which is a continuation of U.S. application Ser. No.15/812,567 filed Nov. 14, 2017, which is a continuation of U.S. patentapplication Ser. No. 15/399,077 filed on Jan. 5, 2017, now issued asU.S. Pat. No. 9,854,239, which is a continuation of U.S. patentapplication Ser. No. 14/368,224, filed on Jun. 23, 2014, now issued asU.S. Pat. No. 9,554,130, which is the National Stage Entry ofInternational Application PCT/KR2013/000370, filed on Jan. 17, 2013, andclaims priority from and the benefit of Korean Patent Application No.10-2012-0006841 filed on Jan. 20, 2012 and Korean Patent Application No.10-2013-0005333 filed on Jan. 17, 2013, the disclosures of which areincorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION Field of the invention

The present invention relates to an intra-prediction mode mapping methodand a device using the method, and more particularly, to an encoding anddecoding method and apparatus.

Related Art

Recently, demand for high resolution, quality images such as highdefinition (HD) images and ultra-high definition (UHD) images hasincreased in various application fields. As video data has increasinglyhigh resolution and high quality, an amount of data is increased,relative to existing video data. Thus, transmission of image data usinga medium such as an existing wired/wireless wideband circuit or storingvideo data using an existing storage medium increases transmission costsand storage costs. In order to address such problems arising as videodata has increasingly high resolution and high quality, highly efficientvideo compression techniques may be utilized.

Video compression techniques may include various techniques such as aninter-prediction technique of predicting a pixel value included in acurrent picture from a previous or subsequent picture of the currentpicture, an intra-prediction technique of predicting a pixel valueincluded in a current picture using pixel information of the currentpicture, an entropy encoding technique of allocating a short code to avalue having high frequency and allocating a long code to a value havinglow frequency, and the like. Video data may be effectively compressedand transmitted or stored through such video compression techniques.

SUMMARY OF THE INVENTION

A first aspect of the present invention provides an intra-predictionmode mapping method to increase video encoding efficiency.

A second aspect of the present provides an apparatus for performing anintra-prediction mode mapping method to increase video encodingefficiency.

In an aspect, an intra-prediction method may include: decoding flaginformation indicating whether one of a plurality of candidateintra-prediction modes with respect to a current block and anintra-prediction mode of the current block are identical; and when oneof a plurality of candidate intra-prediction modes with respect to acurrent block and an intra-prediction mode of the current block are notidentical, decoding a syntax element including information regarding theintra-prediction mode of the current block to derive theintra-prediction mode of the current block, wherein, the decoding of asyntax element including information regarding the intra-prediction modeof the current block to derive the intra-prediction mode of the currentblock is performed based on a table including intra-prediction modeinformation, the table is a table mapping intra-prediction modes andindex information of the intra-prediction modes, and, in the table, whenan intra-prediction mode is a planar mode, the intra-prediction mode ismapped to an index 0, when an intra-prediction mode is a DC mode, theintra-prediction mode is mapped to an index 1, and when intra-predictionmodes are directional intra-prediction modes, the intra-prediction modesare mapped to indices 2 to 34 according to directivity of theintra-prediction modes.

The syntax element including information regarding an intra-predictionmode of a current block may be a value coded using fixed 5 bits, and the5-bit information may indicate one of the remaining intra-predictionmode information, excluding a plurality of candidate intra-predictionmodes with respect to the current block.

The plurality of candidate intra-prediction modes with respect to thecurrent block may be three different intra-prediction modes derivedbased on intra-prediction modes derived based on neighbor blocks of thecurrent block and an additional intra-prediction mode.

When the flag information is 1, the flag information may indicate thatone of the candidate intra-prediction modes of the current block and theintra-prediction mode of the current block are identical, and when theflag information is 0, the flag information may indicate that theplurality of candidate intra-prediction modes of the current block andthe intra-prediction mode of the current block are not identical.

The decoding of a syntax element including information regarding theintra-prediction mode of the current block to derive theintra-prediction mode of the current block may include: deriving theother remaining thirty-two intra-prediction mode information, excludingthree intra-prediction modes as a plurality of candidateintra-prediction modes with respect to the current block, in the table;mapping syntax element including information regarding theintra-prediction mode of the current block to the other remainingthirty-two intra-prediction mode information; and setting oneintra-prediction mode, among the other remaining thirty-twointra-prediction mode information, mapped to the syntax element, as anintra-prediction mode of the current block.

In another aspect, a video decoding apparatus may include: anentropy-decoding module configured to decode flag information indicatingwhether one of a plurality of candidate intra-prediction modes withrespect to a current block and an intra-prediction mode of the currentblock are identical, and configured to decode a syntax element includinginformation regarding the intra-prediction mode of the current block toderive the intra-prediction mode of the current block, when one of aplurality of candidate intra-prediction modes with respect to a currentblock and an intra-prediction mode of the current block are notidentical; and a prediction module configured to derive theintra-prediction mode of the current block based on the decoded syntaxelement, when one of a plurality of candidate intra-prediction modeswith respect to a current block and an intra-prediction mode of thecurrent block are not identical, wherein the table is a table mappingintra-prediction modes and index information of the intra-predictionmodes, and, in the table, when an intra-prediction mode is a planarmode, the intra-prediction mode is mapped to an index 0, when anintra-prediction mode is a DC mode, the intra-prediction mode is mappedto an index 1, and when intra-prediction modes are directionalintra-prediction modes, the intra-prediction modes are mapped to indices2 to 34 according to directivity of the intra-prediction modes.

The syntax element may be a value coded using fixed 5 bits, and the5-bit information may indicate one of the remaining intra-predictionmode information, excluding a plurality of candidate intra-predictionmodes with respect to the current lo block.

The plurality of candidate intra-prediction modes with respect to thecurrent block may be three different intra-prediction modes derivedbased on intra-prediction modes derived based on neighbor blocks of thecurrent block and an additional intra-prediction mode.

When the flag information is 1, the flag information may indicate thatone of the candidate intra-prediction modes of the current block and theintra-prediction mode of the current block are identical, and when theflag information is 0, the flag information may indicate that theplurality of candidate intra-prediction modes of the current block andthe intra-prediction mode of the current block are not identical.

The prediction module may derive the other remaining thirty-twointra-prediction mode information, excluding three intra-predictionmodes as a plurality of candidate intra-prediction modes with respect tothe current block, in the table, map syntax element includinginformation regarding the intra-prediction mode of the current block tothe other remaining thirty-two intra-prediction mode information, andset one intra-prediction mode, among the other remaining thirty-twointra-prediction mode information, mapped to the syntax element, as anintra-prediction mode of the current block.

As described above, the intra-prediction mode mapping method andapparatus using the method according to embodiments of the presentinvention may encode and decode intra-prediction mode information with asmall number of bits, thus increasing video coding efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an encoding apparatus accordingto an embodiment of the present invention;

FIG. 2 is a block diagram illustrating a decoder according to anembodiment of the present invention;

FIG. 3 is a flow chart illustrating a method for decoding anintra-prediction mode of a current prediction module according to anembodiment of the present invention;

FIG. 4 is a conceptual view illustrating a method for encoding anintra-prediction mode according to an embodiment of the presentinvention;

FIG. 5 is a conceptual view illustrating a method for decoding anintra-prediction mode according to an embodiment of the presentinvention;

FIG. 6 is a conceptual view illustrating a case not using a codeNummapping table according to an embodiment of the present invention;

FIG. 7 is a view illustrating non-directional intra-prediction modes anddirectional intra-prediction modes when thirty-five intra-predictionmodes are used according to an embodiment of the present invention;

FIG. 8 is a view illustrating non-directional intra-prediction modes anddirectional intra-prediction modes when nineteen intra-prediction modesare used according to an embodiment of the present invention;

FIG. 9 is a view illustrating non-directional intra-prediction modes anddirectional intra-prediction modes when eleven intra-prediction modesare used according to an embodiment of the present invention;

FIG. 10 is a conceptual view illustrating a codeNum mapping and codemapping method excluding MPM according to an embodiment of the presentinvention;

FIG. 11 is a conceptual view illustrating portions of a video encodingapparatus and a video decoding apparatus respectively performingencoding and decoding of an intra-prediction mode according to anembodiment of the present invention; and

FIG. 12 is a conceptual view illustrating portions of a video encodingapparatus and a video decoding apparatus respectively performingencoding and decoding of an intra-prediction mode according to anembodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Also, elements disclosed in the embodiments and drawings of the presentinvention are independently illustrated to show different characteristicfunctions, and it does not mean that each element is configured asseparated hardware or a single software component. Namely, for the sakeof explanation, respective elements are arranged to be included, and atleast two of the respective elements may be incorporated into a singleelement or a single element may be divided into a plurality of elementsto perform a function, and the integrated embodiment and dividedembodiment of the respective elements are included in the scope of thepresent invention unless it diverts from the essence of the presentinvention.

Also, some of the elements may be optional to merely enhance theperformance, rather than being essential to perform a constitutionalfunction. The present invention may be implemented by using only theelements requisite for implement the essence of the present invention,excluding elements used to merely enhance the performance, and astructure including only the essential elements excluding the optionalelements merely used to enhance the performance is also included in thescope of the present invention.

FIG. 1 is a block diagram illustrating an encoding apparatus accordingto an embodiment of the present invention.

Referring to FIG. 1, an encoding apparatus includes a partition module100, a prediction module 110, an intra-prediction module 103, aninter-prediction module 106, a transformation module 115, a quantizationmodule 120, a rearrangement module 125, an entropy encoding module 130,a dequantization module 135, an inverse-transform module 140, a filtermodule 145, and a memory 150.

The encoding apparatus may be implemented by a video encoding methoddescribed in an exemplary embodiment of the present invention, butoperations of some elements may not be performed for fast real-timeencoding in order to reduce complexity of an encoder. For example, inperforming intra-prediction by the prediction module, rather than amethod of selecting an optimum intra-encoding method using everyintra-prediction mode method to perform encoding in real time, a methodof selecting one of a partial limited number of intra-prediction modes,as a final intra-prediction mode may be used. In another example, inperforming intra-prediction or inter-prediction, prediction blocks maybe limitedly used.

A unit of a block processed by the encoding apparatus may be a codingunit performing encoding, a prediction unit performing prediction, and atransform unit performing transformation. A coding unit may be expressedas CU (coding unit), a prediction unit may be expressed as PU(prediction unit), and a transform unit may be expressed as TU(transform unit).

The partition module 100 may partition a single picture into a pluralityof combinations of a coding block, a prediction block, and a transformblock, and select one of the plurality of combinations of a codingblock, a prediction block and a transform block to split a picture. Forexample, in order to partition coding units (CU) in a picture, arecursive tree structure such as a QuadTree structure may be used.Hereinafter, in an embodiment of the present invention, a meaning of acoding block may be used as a meaning of a block performing decoding, aswell as a meaning of a block performing encoding.

A prediction block may be a unit performing intra-prediction orinter-prediction. The block performing intra-prediction may be a squareblock having a size such as 2N×2N or N×N, or may be a rectangular blockusing short distance intra-prediction (SDIP). In a prediction blocksplitting method, a square block such as 2N×2N or N×N performingintra-prediction, a rectangular block such as 2N×N or N×2N obtained bybisecting a square prediction block to have the same shape, orprediction asymmetric motion partitioning (AMP) having an asymmetricalform may be used. A method for performing transformation by thetransform module 115 may vary depending on a shape of a predictionblock.

The prediction module 110 may include an intra-prediction module 103performing intra-prediction and an inter-prediction module 106performing inter-prediction. Whether to use inter-prediction orintra-prediction on a prediction block may be determined. A processingunit subjected to prediction and a unit of a processing block for whicha prediction method is determined may be different. For example, inperforming intra-prediction, a prediction mode may be determined basedon a prediction block, and a process of performing prediction may beperformed based on a transform block. A residual value (residual block)between a generated prediction block and the original block may be inputto the transform module 115. Also, prediction mode information, motionvector information, or the like, used for prediction may be encoded bythe entropy encoding module 130 together with the residual value anddelivered to the decoder.

In case of using a pulse coded modulation (PCM) coding mode, theoriginal block may be encoded as is and transmitted to the decodingmodule, without performing prediction through the prediction module 110.

The intra-prediction module 103 may generate an intra-predicted blockbased on a reference pixel existing around a current block (block as aprediction target). In order to derive an optimal intra-prediction modewith respect to the current block, an intra-predicted block with respectto the current block may be generated using a plurality ofintra-prediction modes and one of the prediction blocks may beselectively used as a prediction block of the current block. In theintra-prediction method, an intra-prediction mode may include adirectional prediction mode using reference pixel information accordingto a prediction direction and a non-directional mode not usingdirectional information in performing prediction. A mode for predictingluminance information and a mode for predicting chrominance informationmay be different. In order to predict chrominance information,intra-prediction mode information predicting luminance information orpredicted luminance signal information may be utilized.

In case of a current block on which intra-prediction is performed usinga single intra-prediction mode, an intra-prediction mode of the currentblock may be predicted from intra-prediction mode information which hasbeen used in performing intra-prediction on a neighboring block of thecurrent block, and intra-prediction mode information of the currentblock may be encoded. Namely, the intra-prediction mode of the currentblock may be predicted from an intra-prediction mode of a predictionblock existing around the current block. In order to predict anintra-prediction mode of the current block using mode informationpredicted from a neighbor block, the following methods may be used.

1) When an intra-prediction mode of a current block and anintra-prediction mode of a neighbor block are identical, predeterminedflag information may be encoded to transmit information indicating thatan intra-prediction mode of the current block and an intra-predictionmode of the neighbor block are identical.

2) When the intra-prediction mode of the current block and theintra-prediction mode of the neighbor block are different,intra-prediction mode information of the current block may beentropy-encoded to encode prediction mode information of the currentblock.

The intra-prediction mode of the neighbor block used to encode theintra-prediction mode of the current block in 1) and 2) may be definedas a term of a candidate intra-prediction mode and used.

In performing the methods 1) and 2), if the intra-prediction mode of theneighbor block is not available (for example, a neighbor block does notexist or a neighbor block has performed inter-prediction), a pre-setparticular intra-prediction mode value may be set as an intra-predictionmode value to predict an intra-prediction mode of the current block.

The intra-prediction module 103 may generate an intra-predicted blockbased on a reference pixel information around the current block as pixelinformation of a current picture. Since a neighbor block of the currentblock is a block on which inter-prediction has been performed, and thus,a reference pixel is a pixel obtained by restoring a predicted pixel byperformed inter-prediction. In such a case, the current block may beintra-predicted using a pixel of the neighbor block on whichintra-prediction has been performed without using the correspondingpixel. Namely, when the reference pixel is not available, theunavailable reference pixel may be replaced with a different pixel so asto be used.

A prediction block may include a plurality of transform blocks. When asize of a prediction block and a size of a transform block are equal inperform intra-prediction, intra-prediction may be performed on theprediction block based on a pixel existing on the left of the predictionblock, a pixel existing on a left upper side of the prediction block,and a pixel existing in an upper side of the prediction block. However,in a case in which a size of a prediction block and a size of atransform block are different and a plurality of transform blocks areincluded in the prediction block in performing intra-prediction,intra-prediction may be performed using a reference pixel determinebased on the transform blocks.

Also, a single coding block may be partitioned into a plurality ofprediction blocks, and only on a minimum coding block corresponding to acoding block having a minimum size, intra-prediction may be performedusing N×N partitioning by which a single coding block is split into foursquare prediction blocks.

In the intra-prediction method, a prediction block may be generatedafter applying a mode dependent intra smoothing (MDIS) filter on areference pixel according to an intra-prediction mode. Types of MDISfilter applied to the reference pixel may vary. The MDIS filter, anadditional filter applied to an intra-predicted block after performingintra-prediction, may be used to reduce a difference existing betweenthe reference pixel and the intra-predicted block generated afterprediction is performed. In performing MDIS filtering, various filteringmay be performed on a partial row included in the reference pixel andthe intra-predicted block according to directivity of anintra-prediction mode.

According to an embodiment of the present invention, in performingintra-prediction on a current block according to a size of the currentblock, the number of available intra-prediction modes may vary. Forexample, the number of available intra-prediction modes may varyaccording to a size of a current block as a target of intra-prediction.Thus, in performing intra-prediction on a current block, a size of thecurrent block may be determined and available intra-prediction modes maybe determined accordingly to perform intra-prediction.

The inter-prediction module 106 may perform prediction with reference toinformation of a block included in at least one of a previous picture ora subsequent picture of a current picture. The inter-prediction module106 may include a reference picture interpolating module, a motionestimating module, and a motion compensating module.

The reference picture interpolating module may receive reference pictureinformation from the memory 150 and generate pixel information regardinginteger pixel or smaller in the reference picture. In case of aluminance pixel, DCT-based 8-tap interpolation filter using a filterfactor varied to generate pixel information regarding an integer pixelor smaller in units of ¼ pixel. In case of a chrominance signal, aDCT-based 4-tap interpolation filter using a filter factor varied togenerate pixel information regarding an integer pixel or smaller inunits of ⅛ pixel.

The inter-prediction module 106 may perform motion prediction based on areference picture interpolated by the reference picture interpolatingmodule. In order to calculate a motion vector, various methods such asfull search-based block matching algorithm (FBMA), three step search(TSS), new tree-step search algorithm (NTS), or the like, may be used. Amotion vector may have a motion vector value in units of ½ or ¼ pixelbased on an interpolated pixel. The inter-prediction module 106 mayperform prediction on a current block by applying one of variousinter-prediction methods. The inter-prediction methods may include, forexample, a skip method, a merge method, an advanced motion vectorprediction (AMVP) method, or the like.

A residual block including residual information as a difference valuebetween a generated predicted block (intra-predicted block orinter-predicted block) which has been generated by the prediction module110 and the original block may be generated.

The generated residual block may be input to the transform module 115.The transform module 115 may transform the original block and theresidual block including the residual information of the predicted blockthrough a transform method such as discrete cosine transform (DCT) ordiscrete sine transform (DST). Whether to apply DCT or DST to transformthe residual block may be determined based on intra-prediction modeinformation of the prediction block used to generate the residual blockand size information of the prediction block. Namely, the transformmodule 115 may differently apply a transform method according to a sizeof a prediction block and a prediction method.

The quantization module 120 may quantize values which have beentransformed into a frequency domain by the transform module 115. Aquantization coefficient may vary according to a block or importance ofan image. A value calculated by the quantization module 120 may beprovided to the dequantization module 135 and the rearrangement module125.

The rearrangement module 125 may rearrange a coefficient value withrespect the quantized residual value. The rearrangement module 125 maychange a two-dimensional (2D) block type coefficient into aone-dimensional (1D) vector form through a coefficient scanning method.For example, the rearrangement module 125 may scan from a DC coefficientup to a coefficient of a high frequency domain by using a zigzag scanmethod to change them into a 1D vector form. According to a size of atransform unit and an intra-prediction mode, a vertical scan method ofscanning 2D block type coefficients in a column direction or ahorizontal scan method of scanning 2D block type coefficients in a rowdirection, rather than the zigzag scan method, may be used. Namely,which one of the zigzag scan method, the vertical scan method, and thehorizontal scan method is to be used may be determined according to asize of a transform unit and an intra-prediction mode.

The entropy encoding module 130 may perform entropy encoding based onthe values calculated by the rearrangement module 125. For example,various encoding methods such as exponential Golomb, context-adaptivevariable length coding (CAVLC), and context-adaptive binary arithmeticcoding (CABAC) may be used as entropy encoding.

The entropy encoding module 130 may receive various types of informationsuch as residual value coefficient information of a coding block andblock type information, prediction mode information, partitioning unitinformation, prediction block information and transmission unitinformation, motion vector information, reference frame information,block interpolation information, filtering information, and the like,from the rearrangement module 125 and the prediction module 110, andperform entropy encoding thereon based on a predetermined coding method.Also, the entropy encoding module 130 may entropy-encode a coefficientvalue of a coding unit input from the rearrangement module 125.

The entropy encoding module 130 may perform entropy coding through ahigh efficiency binarization (HEB) method using CABAC or a highthroughput binarization (HTB) method utilizing CAVLC factor binarizationmethod for bypass coding of CABAC.

The entropy-encoding module 130 performs binarization onintra-prediction mode information to encode the intra-prediction modeinformation of the current block. The entropy-encoding module 130 mayinclude a codeword mapping module for performing such a binarizationoperation, and binarization may be performed variously according to asize of a prediction block performing intra-prediction. The codewordmapping module may adaptively generate a codeword mapping table throughbinarization operation or may have a codeword mapping table storedtherein in advance. In another embodiment, the entropy-encoding module130 may express the intra-prediction mode information of the currentblock using a codeNum mapping module performing codeNum mapping and acodeword mapping module performing codeword mapping. The codeNum mappingmodule and the codeword mapping module may generate a codeNum mappingtable and a codeword mapping table, or may have a codeNum mapping tableand a codeword mapping table stored therein.

The dequantization module 135 and the inverse-transform module 140 maydequantize the values quantized by the quantization module 120 andinverse-transforms values transformed by the transform module 115.Residual values generated by the dequantization module 135 and theinverse-transform module 140 may be added with the prediction blockpredicted through the motion estimation module, the motion compensationmodule, and the intra-prediction module included in the predictionmodule to generate a reconstructed block.

The filter module 145 may include at least one of a deblocking filter,an offset compensation module, and an adaptive loop filter (ALF).

The deblocking filter 145 may remove block distortion generated due to aboundary between blocks in the reconstructed picture. In order todetermine whether to perform deblocking, whether to apply a deblockingfilter to a current block based on pixels included in some columns androws included in a block may be determined. In a case in which thedeblocking filter is applied to the block, a strong filter or a weakfilter may be applied according to strength of required deblockingfiltering. Also, in applying the deblocking filter, when verticalfiltering and horizontal filtering are performed, horizontal directionalfiltering and vertical directional filtering may be processedconcurrently.

The offset compensation module may compensate for an offset with anoriginal image by pixels with respect to a deblocked image. In order toperform offset compensation on a specific picture, a method of dividingpixels included in an image into a predetermined number of regions,determining a region in which offset is to be performed, and applyingoffset to the corresponding region, or a method of applying offset inconsideration of edge information of each pixel may be used.

The ALF may perform filtering based on a value obtained by comparing thefiltered reconstructed image and the original image. Pixels included inan image may be divided into a predetermined group, a filter to beapplied to a corresponding group may be determined to differentiallyfiltering each group. Information regarding whether to apply the ALF maybe transmitted by coding unit (CU), and a size and a coefficient of theALF to be applied may be vary according to each block. The ALF may havevarious shapes, and the number of coefficients included in the filtermay also vary. Filtering-related information (filter coefficientinformation, ALF ON/OFF information, filter shape information) of theALF may be included in a form of bit stream in a predetermined parameterset and transmitted.

The memory 150 may store a reconstructed block or picture calculatedthrough the filter module 145, and the stored reconstruction block orpicture may be provided to the prediction module 110 wheninter-prediction is performed.

FIG. 2 is a block diagram illustrating a decoder according to anembodiment of the present invention.

Referring to FIG. 2, the decoder may include an entropy decoding module210, a rearrangement module 215, a dequantization module 220, aninverse-transform module 225, a prediction module 230, a filter module235, and a memory 240.

When a bit stream is input from an encoder, the input bit stream may bedecoded in a procedure opposite to that of the encoder.

The entropy decoding module 210 may perform entropy decoding in aprocedure opposite to that of the entropy coding by the entropy codingmodule of the encoder. Information for generating a prediction block,among the information decoded by the entropy decoding module 210 may beprovided to the prediction module 230, and the residual value obtainedafter performing the entropy decoding by the entropy decoding module maybe input to the rearrangement module 215.

Like the entropy encoding module, the entropy decoding module 210 mayperform inverse-transform using at least one of the HEB method usingCABAC and HTB method utilizing a CALVAC factor coding method.

The entropy decoding module 210 may decode information related tointra-prediction and inter-prediction performed by the encoder. Theentropy-encoding module may include a codeword mapping module to includea codeword mapping table for generating a received codeword as anintra-prediction mode number. The codeword mapping table may be storedin advance or may be adaptively generated. In case of using a codeNummapping table, a codeNum mapping module may be further provided toperform codeNum mapping.

The rearrangement module 215 may perform rearranging based on the methodof rearranging an entropy-decoded bit stream by the entropy decodingmodule 210. Coefficients expressed in the form of a 1D vector may bereconstructed into coefficients in the form of a 2D block and realigned.The rearrangement module 215 may be provided with information related tocoefficient scanning performed in the encoding module, and performrearrangement through a method of reversely scanning based on thescanning order performed in the corresponding encoding module.

The dequantization module 220 may perform dequantization based on aquantization parameter provided from the encoder and the realignedcoefficient value of the block.

The inverse-transform module 225 may perform inverse-DCT and inverse-DSTon the DCT and the DST performed by the transform module on thequantization results performed by the encoder. The inverse-transform maybe performed based on a transmit unit determined by the encoder. Thetransform module 115 of the encoder may selectively perform DCT and DSTaccording to a plurality of information such as a prediction method, asize of a current block, a prediction direction, and the like, and thedequantization module 225 of the decoder may perform inverse transformbased on the information transformed by the transform module of theencoder.

When the transform is performed, the transform may be performed based oncoding unit (CU), rather than based on the transform unit (TU).

The prediction module 230 may generate a prediction block based on theprediction block generation-related information provided from theentropy decoding module 210 and the previously decoded block or pictureinformation provided from the memory 240.

As described above, like the operation in the encoder, in performingintra-prediction, when a size of a prediction block and a size of atransform block are equal, intra-prediction is performed on a predictionblock based on pixels existing on the left of the prediction block,pixels existing in left upper side of the prediction block, and pixelsexisting in an upper side of the prediction block. However, inperforming intra-prediction, when a transform block is included in aprediction block, intra-prediction may be performed by using a referencepixel based on the transform block. Also, as described above,intra-prediction using N×N partitioning only on a minimum coding blockmay be used.

The prediction module 230 may include a prediction unit discriminatingmodule, an inter-prediction module, and an intra-prediction module. Theprediction unit discriminating module may receive various types ofinformation such as prediction unit information input from the entropydecoding module, prediction mode information of an intra-predictionmethod, a motion prediction-related information of an inter-predictionmethod, and the like, input from the entropy decoding module,discriminate prediction block from a current coding block, anddiscriminate whether a prediction block is to perform inter-predictionor intra-prediction. The inter-prediction module may performinter-prediction on a current prediction block based on informationincluded in at least one picture among a previous picture and asubsequent picture of a current picture including a current predictionblock by using information required for inter-prediction of the currentprediction block provided from the encoder.

In order to perform inter-prediction, based on a coding block, which oneof a skip mode, a merge mode, and an AMVP mode corresponds to a motionprediction method of a prediction block included in the correspondingcoding block may be determined.

The intra-prediction module may generate a prediction block based onpixel information of the current picture. In a case in which aprediction block is a prediction block which has performedintra-prediction, intra-prediction may be performed based onintra-prediction mode information of the prediction block provided fromthe encoder. The intra-prediction module may include an MDIS filter, areference pixel interpolation module, and a DC filter. The MDIS filteris a part performing filtering on a reference pixel of a current block,and whether to apply the filter may be determined according to aprediction mode of a current PU and applied. MDIS filtering may beperformed on the reference pixel of the current block by using theprediction mode of the prediction block and the MDIS filter informationprovided from the encoder. In a case in which the prediction mode of thecurrent block is a mode in which MDIS filtering is not performed, theMDIS filter may not be applied. Also, like the encoder, after generatinga prediction block, filtering may be performed additionally togetherwith a reference pixel.

When the prediction mode of the prediction block is a prediction blockthat performs intra-prediction based on a pixel value obtained byinterpolating the reference pixel, the reference pixel interpolationmodule may interpolate the reference pixel to generate a reference pixelof a pixel unit below an integer value. In a case in which theprediction mode of the current prediction block is a prediction mode inwhich a prediction block is generated without interpolating thereference pixel, the reference pixel may not be interpolated. In a casein which the prediction mode of the current block is a DC mode, the DCfilter may generate a prediction block through filtering.

The reconstructed block or picture may be provided to the filter module235. The filter module 235 may include a deblocking filter, an offsetcompensation module, and an ALF.

Information regarding whether a deblocking filter has been applied tothe corresponding block or picture and information regarding whether astrong filter or a weak filter has been applied if the deblocking filterhas been applied may be received from the encoder. The deblocking filterof the decoder may receive deblocking filter-related informationprovided form the encoder and the decoder may perform deblockingfiltering on the corresponding block. Like in the encoder, first,vertical deblocking filtering and horizontal deblocking filtering may beperformed, and at least one of vertical deblocking and horizontaldeblocking may be performed in an overlap portion. Vertical deblockingfiltering or horizontal deblocking filtering which has not beenperformed previously may be performed in a portion in which verticaldeblocking filter and horizontal deblocking filtering overlap. Throughthe deblocking filtering process, parallel processing of the deblockingfiltering may be performed.

The offset compensation module may perform offset compensation on thereconstructed image based on a type of offset compensation applied tothe image during encoding, offset value information, and the like.

ALF may perform filtering based on a value obtained by comparing theimage reconstructed after filtering and the original image. Based oninformation whether ALF has been applied, ALF coefficient information,and the like, provided from the encoder, ALF may be applied to a CU.Such ALF information may be included in a specific parameter set andprovided.

The memory 240 may store the reconstructed picture or block so as to beused as a reference picture or reference block, and or provide thereconstructed picture to the output module.

As described above, in the embodiment of the present invention, a codingunit (CU) is used as a term of coding block, but it may also be a blockof performing decoding as well as encoding. Hereinafter, anintra-prediction method according to an embodiment of the presentinvention described with reference to FIGS. 3 through 12 may beimplemented according to functions of each mode described above withreference to FIGS. 1 and 2 and such encoder and decoder may fall withinthe scope of the present invention.

An intra-prediction mode of a current prediction block may be predictedfrom an intra-prediction mode of a neighbor block. Such a predictionmethod of an intra-prediction mode is called a most probable mode (MPM).A first MPM and a second MPM may be set based on an intra-predictionmode of a block existing on the left and in an upper side of the currentblock or an intra-prediction mode used frequently, and when anintra-prediction mode of the current block is identical to at least oneof intra-prediction modes of the first MPM and the second MPM,information prev_intra_pred_flag indicating that the intraintra-prediction mode of the current block is identical to theintra-prediction mode of the MPM is used, and to which of theintra-prediction modes among the first MPM and the second MPM theintra-prediction mode of the prediction block is identical may beindicated through mpm_idx information. When the intra-prediction mode ofthe MPM is not identical to the intra-prediction mode of the currentblock, information regarding the intra-prediction mode of the currentblock may be encoded with rem_intra_luma_pred_mode.

FIG. 3 is a flow chart illustrating a method for decoding anintra-prediction mode of a current prediction unit according to anembodiment of the present invention.

Referring to FIG. 3, the foregoing prev_intra_pred_flag information maybe decoded to decode intra-prediction mode information of the currentblock (step S300).

It is determined whether the decoded prev_intra_pred_flag information is1 or 0 (step S300).

In an embodiment of the present invention, when the prev_intra_pred_flaginformation is 1, it is determined that the MPM and the intra-predictionmode of the current block are identical, and when theprev_intra_pred_flag information is 0, it is determined that the MPM andthe intra-prediction mode of the current block are different. However,any other binarization method or information display method with respectto the prev_intra_pred_flag information may also be used.

When the intra-prediction mode of the current block and theintra-prediction mode of the MPM are determined to be identical throughthe prev_intra_pred_flag, the mpm_idx may be decoded to obtaininformation regarding to which of intra-prediction modes of the firstMPM and the second MPM the intra-prediction mode of the current block isidentical (step S320). When it is determined that the intra-predictionmode of the current block and the intra-prediction mode of the MPM arenot identical through the prev_intra_pred_flag,rem_intra_luma_pred_mode, remaining mode information, may be decoded toobtain intra-prediction mode information of the current block (stepS330).

In order to encode/decode the rem_intra_luma_pred_mode information,codeword information mapped to the intra-prediction mode may be used.Table 1 shows Exp-Golomb encoding method, one of methods of binarizingintra-prediction mode information.

TABLE 1 codeword codenum 1 0 010 1 011 2 00100 3 00101 4 00110 5 . . . .. .

Referring to Table 1, as the code numbers are smaller on the codewordmapping table, they may be mapped to shorter codewords. Namely, it canbe seen that, when a short codeword is mapped to frequently generatedinformation, the same information may be expressed with a shorter bitstream, and thus, encoding/decoding efficiency may increase.

Table 2 shows order of intra-prediction modes according to an embodimentof the present invention. Table 2 may be expressed like Table 3.

TABLE 2 Intra prediction mode Associated names 0 Intra_Planar 1 Intar_DC2 Intra_Vertical 3 Intra_Hotizontal otherwise (4 . . . 34) Intra_Angular

TABLE 3 Intra prediction mode Associated names 0 Intra_Planar 1 Intar_DCotherwise (2 . . . 34) Intra_Angular

Table 4 shows mapping between intra-prediction modes and codewords. InFIG. 4, codewords are randomly set and information regarding anintra-prediction mode of a current picture may be mapped using any othercodewords within the scope of the essence of the present invention.

TABLE 4 Intra prediction mode Associated names Codeword 0 Intra_Planar 11 Intar_DC 010 2 Intra_Horizontal 011 3 Intar_Vertical 00100 otherwise(4 . . . 34) Intra_Angular . . .

Referring to Table 4, in the method of mapping intra-prediction modesand codewords according to an embodiment of the present invention,shorter codewords may be mapped to frequently generated intra-predictionmodes, and thus, frequently generated intra-prediction modes may haveshorter codewords.

Hereinafter, a shortest codeword or first mapped codeword based ongenerated order may be defined as a first codeword, and second, third,fourth, and nth codewords, or the like, may be expressed in sequence.Namely, a length of the nth codeword may be shorter than or equal tothat of (n+1)th codeword, but the length of the nth codeword may not begreater than that of the (n+1)th codeword (here, n is an integer).

In case of coding according to an intra-prediction method, a planer modeor a DC mode, non-directional intra-prediction mode amongintra-prediction modes, are frequently used, while a directionalintra-prediction mode occurs relatively unfrequently. Thus, in anembodiment of the present invention, a shorter codeword is mapped to anon-directional intra-prediction mode and a codeword longer than acodeword mapped to a non-directional intra-prediction mode is mapped toa directional intra-prediction mode, thereby increasing video encodingand decoding efficiency.

Table 5 and Table 6 show various binary coding methods according tointra-prediction modes according to an embodiment of the presentinvention.

TABLE 5 Intra prediction mode Prefix (unary code) Remainder (fixedlength) 0-1 0 x (1 bit) 2-5 10 xx (2 bits) 6-9 110 xx (2 bits) 10-18 111xxx (3 bits)

TABLE 6 Intra prediction mode Prefix (unary code) Remainder (fixedlength) 0-1 0 x (1 bit) 2-5 10 xx (2 bits)  6-13 110 xx (2 bits) 14-211110 xxx (3 bits) 22-34 1111 xxxx (4 bits)

Table 5 shows a binarization method for expressing intra-predictionmodes in case of using 19 modes as intra-prediction modes, and Table 6shows a binarization method for expressing intra-prediction modes incase of using 35 modes as intra-prediction modes.

Referring to Table 5 and Table 6, a unary code and a fixed length may beused as a binary coding method for expressing intra-prediction modes ofa current block. Referring to Table 5, when an intra-prediction mode is0 or 1, a prefix using a unary code may be fixed to 0 and a fixed lengthfor discriminating between 0 and 1, intra-prediction modes, may beexpressed by 1 bit with 0 or 1. Also, when intra-prediction modes are 2to 5, a prefix using a unary code may be fixed to 10 and a fixed lengthfor discriminating among the intra-prediction modes 2 to 5 may beexpressed by 2 bits. In this manner, codewords and intra-predictionmodes may be mapped using a unary code and a fixed length. Table 6 mayalso binarize intra-prediction mode information in this manner toexpress the same.

Table 5 and Table 6 may also use a method in which a number of anintra-prediction mode is smaller, it is generated as a shorter codewordwhen binarized. Thus, according to an embodiment of the presentinvention, by disposing a frequently generated intra-prediction mode inan intra-prediction mode having a smaller number, correspondinginformation may be expressed with a small amount of bits, increasingcoding efficiency.

FIG. 4 is a conceptual view illustrating a method for encoding anintra-prediction mode according to an embodiment of the presentinvention.

Referring to FIG. 4, according to an embodiment of the presentinvention, a codeNum mapping table may be used in encoding anintra-prediction mode.

Unlike the embodiment of Table 5 or Table 6 as described above, in caseof using a codeNum mapping table, when a certain intra-prediction modeis determined, t-th determined intra-prediction mode and (t−1)thintra-prediction mode in the codeNum mapping table are swapped to raisea ranking of the frequently generated intra-prediction mode in thecodeNum mapping table, whereby when a subsequent intra-prediction modeis positioned in the frequently generated intra-prediction mode, asmaller number of codeNum may be allocated. As a result, a shortercodeword may be mapped to the corresponding codeNum. Namely, the codeNummapping table rearranged by raising the codeNum ranking of the t-thintra-prediction mode may be used when an intra-prediction is performedon a next prediction block. Referring to FIG. 4, the determinedintra-prediction mode is 2, and codeNum 2 mapped to the #2intra-prediction mode is read from the codeNum mapping table and acodeword (“1000”) mapped to the #2 codeNum in the codeword mapping tableis output as a result value of the #2 intra-prediction mode. Also, inthe codeNum mapping table, the #2 intra-prediction mode and theimmediately upper #1 intra-prediction mode are swapped, so that thecodeNum for the #2 intra-prediction mode is corrected to #1 and thecodeNum for the #1 intra-prediction mode is corrected to #2. Therearranged codeNum mapping table is used to intra-prediction mode codingof a next prediction block. In another embodiment, whether to swap t-thand (t−1)th intra-prediction modes is determined based on frequency ofeach of the t-th and (t−1)th intra-prediction modes. Namely, when thefrequency of the t-th intra-prediction mode is greater than that of the(t-1)th intra-prediction mode, the t-th and (t−1)th intra-predictionmodes are swapped and the rearranged codeNum mapping table may be usedwhen intra-prediction is performed on a next prediction block.Conversely, when the frequency of the t-th intra-prediction mode issmaller than that of the (t−1)th intra-prediction mode, the twointra-prediction modes are not swapped and a current codeNum mappingtable may be used when intra-prediction is performed on a nextprediction block. In this case, in order to prevent a frequency of everyintra-prediction mode from increasing infinitely, frequency of everyintra-prediction mode may be reduced at the same rate periodically. Aprocess (for example, for determining a codeword) other than theswapping process with respect to an intra-prediction mode based onfrequency may be the same as the foregoing embodiment (namely, swappingimmediately without using frequency).

In FIG. 4, for the purposes of description, it is assumed thatintra-prediction mode numbers and codeNum numbers of the codeNum mappingtable are identical, but in another embodiment, an initial codeNummapping table mapping intra-prediction mode numbers and codeNum may bepredetermined and codeNum numbers different from those ofintra-prediction modes may be mapped to intra-prediction modes.

Referring to FIG. 4, when a certain intra-prediction mode is determined,the intra-prediction mode may be mapped to a codeNum value of thecodeNum mapping table. The mapped codeNum value may be mapped to acodeword value through a codeword mapping table.

FIG. 5 is a conceptual view illustrating a method for decoding anintra-prediction mode according to an embodiment of the presentinvention.

Referring to FIG. 5, the decoder has an initial codeword mapping tableand an initial codeNum mapping table identical to those of the encoder.The decoder reads a codeword from a bit stream and obtains a mappedcodeNum value from the codeword mapping table based on the correspondingcodeword, and when an intra-prediction mode mapped to the codeNum isobtained from the codeNum mapping table, the decoder may finallydetermine an intra-prediction mode of a current block. During thedecoding operation, when a certain intra-prediction mode is determinedin the same manner as that of the encoder, swapping may be performed onthe intra-prediction mode in the codeNum mapping table.

When the foregoing method is performed, in order to prevent unnecessaryswapping, a value of the initial codeNum mapping table is important. Thereason is because such a table may be re-initialized in a certain unitsuch as a slice or a frame, swapping needs to be performed afterinitialization to generate a codeNum mapping table reflecting thecharacters of a current slice or frame. Thus, it is important to map asmaller codeNum value to a frequently generated intra-prediction modevalue in configuring a codeNum mapping table mapping intra-predictionmodes and codeNum values. According to an embodiment of the presentinvention, a smaller codeNum number is mapped to a more frequentlygenerated intra-prediction mode number, and as a result, a length of acodeword with respect to the intra-prediction mode may be reduced toobtain coding efficiency.

FIG. 6 is a conceptual view illustrating a case using a codeNum mappingtable differently according to an embodiment of the present invention.

Referring to FIG. 6, in binary-coding an intra-prediction mode,binary-coding may be performed without using an MPM. Namely, a method ofrearranging codeNum mapping table based on intra-prediction modeinformation of blocks existing on the left and in an upper side of aspecific prediction block, without generating flag information for anMPM, may be used. For example, when an intra-prediction mode of blocksexisting on the left and in the upper side are 2 and 3, respectively, amethod of configuring a codeNum mapping table in which 2 and 3 arepositioned in an upper portion of the codeNum mapping table and theother remaining intra-prediction modes are backed may be used. Namely, amethod in which intra-prediction modes of the blocks existing on theleft and the upper side are positioned in upper portion on the codeNummapping table, and thereafter, the other remaining intra-predictionmodes, excluding the intra-prediction modes of the blocks existing onthe left and the upper side, are disposed on the codeNum mapping tablemay be used to generate a codeword corresponding to the intra-predictionmode.

In the foregoing codeNum mapping table rearranging method, it is on thepremise that an intra-prediction mode of a specific prediction block ishighly likely to be identical to at least one of intra-prediction modesof the blocks existing on the left and/or the upper side of the specificprediction block. Thus, rearranging of the foregoing codeNum mappingtable may be minimized by disposing a relatively frequently generatedintra-prediction mode in an upper portion of the codeNum mapping table.Here, the frequently generated intra-prediction mode may be anintra-prediction mode of the blocks existing on the left and/or theupper side of the specific prediction block or may not. If thefrequently generated intra-prediction mode is an intra-prediction modeof blocks existing on the left and/or upper side of the specificprediction block, a probability of rearrangement is relatively reduced,preventing unnecessary rearrangement. Conversely, if the frequentlygenerated intra-prediction mode is not an intra-prediction mode ofblocks existing on the left and/or upper side of the specific predictionblock, when the codeNum mapping table is rearranged by positioning anintra-prediction mode of the blocks existing on the left and/or upperside of the specific prediction block in an upper portion of the codeNummapping table and backing the other remaining intra-prediction modes, afrequently generated intra-prediction mode among the other remainingintra-prediction modes needs to be mapped to a relatively small codeNumin order to be allocated a shorter codeword. As a result, no matterwhether the frequently generated intra-prediction mode is identical tothe intra-prediction modes of the blocks existing on the left and/or theupper side of the specific prediction block or not, it may beadvantageous for the frequently generated intra-prediction mode to beallocated a smaller codeNum number, in terms of compression performanceand/or complexity.

In another embodiment, in binary-coding an intra-prediction mode,binary-coding may be performed using an MPM. However, a codeword withrespect to an MPM may be allocated using a method of rearranging acodeNum mapping table based on intra-prediction mode information of afirst MPM and a second MPM of a specific prediction block withoutgenerating flag information for the MPMs. For example, whenintra-prediction modes of the first MPM and the second MPM are 2 and 3,respectively, a codeNum mapping table may be configured by positioning 2and 3 in an upper portion of the codeNum mapping table and backing otherintra-prediction modes. Other processes may be the same as those of theembodiment of positioning the intra-prediction mode of the blocksexisting on the left and the upper side of a specific prediction blockin an upper portion of a codeNum mapping table.

According to an embodiment of the present invention, different number ofintra-prediction modes may be used according to a size of a predictionblock. Table 7 show the number of intra-prediction modes that may beused according to sizes of a prediction block.

TABLE 7 log2PUSize intraPredModeNum 2 (4 × 4) 11, 18, or 19 3 (8 × 8) 354 (16 × 16) 35 5 (32 × 32) 35 6 (64 × 64) 11, 18, or 19

Referring to Table 7, when a size of a prediction block is 4×4 or 64×64,eleven, eighteen, nineteen intra-prediction modes may be used. Also,when sizes of a prediction block are 16×16, 32×32, and 64×64,thirty-five intra-prediction modes may be used.

Thirty-five intra-prediction modes may have intra-prediction modenumbers and corresponding names as shown in Table 8 below.

TABLE 8 Intra Intra Prediction Prediction mode angle 0 Planar 1 DC 2 Ver3 Hor 4 Ver−8 5 Ver−4 6 Ver+4 7 Ver+8 8 Hor−4 9 Hor+4 10 Hor+8 11 Ver−612 Ver−2 13 Ver+2 14 Ver+6 15 Hor−6 16 Hor−2 17 Hor+2 18 Hor+6 19 Ver−720 Ver−5 21 Ver−3 22 Ver−1 23 Ver+1 24 Ver+3 25 Ver+5 26 Ver+7 27 Hor−728 Hor−5 29 Hor−3 30 Hor−1 31 Hor+1 32 Hor+3 33 Hor+5 34 Hor+7

FIG. 7 is a view illustrating non-directional intra-prediction modes anddirectional intra-prediction modes when 35 intra-prediction modes areused according to an embodiment of the present invention.

In a case in which 35 intra-prediction modes are used to performintra-prediction on a prediction block, the modes may be planar, DC,Ver+x (x is an integer between −8 to 8), or Hor+x (x is an integer from−7 to 8).

FIG. 8 is a view illustrating non-directional intra-prediction modes anddirectional intra-prediction modes when 19 intra-prediction modes areused according to an embodiment of the present invention.

In a case in which nineteen intra-prediction modes are used to performintra-prediction on a prediction block, the modes may be planar, DC,Ver+2x (x is an integer between −4 to 4), or Hor+2x (x is an integerfrom −3 to 4). Namely, unlike the case of using 35 intra-predictionmodes in FIG. 7, in case of using the 19 intra-prediction modes, amethod of selecting only intra-prediction modes with respect to an anglemovement by multiple of 2 each time in vertical and horizontaldirections.

In case of using 18 intra-prediction modes, a method of performingintra-prediction using intra-prediction modes from 0 to 17, excluding 18intra-prediction mode, may be used.

FIG. 9 is a view illustrating non-directional intra-prediction modes anddirectional intra-prediction modes when 11 intra-prediction modes areused according to an embodiment of the present invention.

In case of using 11 intra-prediction modes, the modes may be planar, DC,Ver+4x (x is an integer between −2 and 2), or Hor+4x (x is an integerbetween −1 to 2). Namely, unlike the case of using 35 intra-predictionmodes in FIG. 7, in case of using the 11 intra-prediction modes, amethod of selecting only intra-prediction modes with respect to an anglemovement by multiple of 4 each time in vertical and horizontaldirections. In case of the 11 intra-prediction modes, a correspondingspace is quartered based on VER (mode 2) and HOR (mode 3) to generate 9modes from mode 4 to mode 10 as illustrated in FIG. 9. When the planar(mode 0) and the DC mode (mode 1) are added to the corresponding 9modes, a total of 11 modes may be configured.

As illustrated in FIGS. 8 and 9, 11 intra-prediction modes and 19intra-prediction modes may be used in a 64×64 block or 4×4 block.

When a size of a prediction block is 64×64, it may mean that there is noneed to partition the corresponding block into smaller (for example,32×32, 16×16, or the like), and it may be determined that there is nosignificant change in pixel values of the corresponding block. Thus,when the entire 35 modes are evaluated over the flat block, predictionvalues with respect to the most intra-prediction modes are obtained tobe similar, not significantly making a difference in performance of eachmode having similar directivity. Thus, when complexity is taken intoconsideration, it may be advantageous to perform intra-prediction basedon only some intra-prediction modes, rather than evaluating all of the35 modes, in terms of complexity. Thus, in an embodiment of the presentinvention, as illustrated in FIGS. 8 and 9, inter-prediction may beperformed using any one of the 11 intra-prediction modes, 18intra-prediction modes, and 19 intra-prediction modes with respect to a64×64 prediction block.

Also, in case of a prediction block having a size of 4×4, since the sizeof the prediction block is small, prediction values of the 35intra-prediction modes may be similar, and thus, inter-prediction may beperformed using one of 11 intra-prediction modes, 18 intra-predictionmodes, and 19 intra-prediction modes as in the prediction block havingthe size of 64×64, rather than using all of the 35 intra-predictionmodes.

As described above, in the case in which 11 intra-prediction modes, 19intra-prediction modes, or 35 intra-prediction modes are used and thenumber of MPM modes is 3, coding efficiency may be increased bygenerating a codeNum mapping table with respect to a remaining modeexcluding the MPMs.

Hereinafter, an MPM used in an embodiment of the present invention isused as having a general concept of a candidate intra-prediction modepredicting an intra-prediction mode value of a current block, and a mostprobable remaining mode (MPRM) having a similar concept may also be usedand included in the scope of the present invention, but only an MPM willbe described for the purposes of description.

FIG. 10 is a conceptual view illustrating a codeNum mapping method withrespect to remaining intra-prediction mode excluding MPM according to anembodiment of the present invention.

Referring to FIG. 10, in a case in which MPMs are 2, 4, and 5 asintra-prediction mode numbers, codeNum mapping may be performed on theother remaining intra-prediction modes excluding 2, 4, and 5, in thecodeNum mapping table. Namely, in remaining mode coding, anintra-prediction mode corresponding to an MPM is not selected, and thus,codeNum values may be assigned only to remaining modes excluding theMPMs. The codeNum values may be mapped to codewords by using a codewordmapping table. In this manner, codewords are mapped only tointra-prediction modes corresponding to remaining modes, an unnecessarywaste of codewords may be prevented, increasing coding efficiency.

Similarly, in case of performing a decoding operation, codeNum may begenerated based on an input codeword, and intra-prediction modeinformation may be decoded using a codeNum mapping table in whichintra-prediction modes, excluding intra-prediction modes correspondingto MPMs, are mapped.

FIG. 10 illustrates a case of using 11 intra-prediction modes.intra-prediction is performed on a prediction block using 11intra-prediction modes and 3 MPMs are used, codewords may be mapped to 8remaining intra-prediction modes. When the remaining intra-predictionmodes are expressed by a fixed length, codewords with respect toremaining 8 intra-prediction modes may be mapped by a 3-bit code length.

Table 9 below shows remaining intra-prediction modes by a fixed lengthaccording to an embodiment of the present invention.

TABLE 9 codeNum Codeword 0 000 1 001 2 010 3 011 4 100 5 101 6 110 7 111

As illustrated in FIG. 9, code numbers may be mapped to codewords usingfixed length coding.

In this manner, when the number of intra-prediction modes are 11, 19,and 35, and the number of MPMs is 3, the number of remaining modes are8, 16, and 32, and thus, the total number of the entire remaining modesis exponentiation of 2, and the remaining modes are expressed by fixedlengths of respective exponent bits. Table 10 show lengths of codewordsindicating remaining modes when code numbers are expressed by fixedlengths of exponent bits according to the number of remaining modes.

TABLE 10 IntraPredModeNum Remaining node number Code length 11  8 (2³) 3bits 19 16 (2⁴) 4 bits 35 32 (2⁵) 5 bits

Referring to Table 10, in a case in which the number of intra-predictionmodes is 11 and the number of MPMs is 3, 3-bit codeword representing aremaining mode may be used. In a case in which the number ofintra-prediction modes is 19 and the number of MPMs is 3, 4-bit codewordrepresenting a remaining mode may be used. In a case in which the numberof intra-prediction modes is 35 and the number of MPMs is 3, 5-bitcodeword representing a remaining mode may be used. Namely, according toan embodiment of the present invention, the number of intra-predictionmodes of a current block may be generated by an exponentiation of 2 ofthe number of corresponding remaining intra-prediction modes, namely, itmay be determined that the remaining modes are expressed by fixedlengths of respective exponent bits. In the foregoing embodiment, thenumber of MPMs is assumed to be 3, and when the number of MPMs isdifferent, the number of intra-prediction modes used forintra-prediction of a prediction block may be changed.

FIG. 11 is a conceptual view illustrating portions of a video encodingapparatus and a video decoding apparatus respectively performingencoding and decoding of an intra-prediction mode according to anembodiment of the present invention.

Referring to the upper portion in FIG. 11, a codeword mapping module1100 of a video encoding apparatus may receive an intra-prediction modenumber and generate a codeword. In order to perform codeword mapping asdescribed above, the codeword mapping module 1100 may have a codewordmapping table stored therein or may perform a binarization operation togenerate a codeword mapping table. The codeword mapping module 1100 maybe included in a entropy coding module and operate. As described above,a codeword mapping table may be generated such that a smallintra-prediction mode number may be given to a non-directionalintra-prediction mode such as a DC mode or a planar mode having a highincidence among intra-prediction modes and a shorter codeword is mappedto a smaller intra-prediction mode number.

Referring to a lower portion in FIG. 11, a codeword mapping module 1110of a video decoding apparatus receives a codeword and generate the inputcodeword as an intra-prediction mode number based on a codeword mappingtable included in the codeword mapping module 1110.

FIG. 12 is a conceptual view illustrating portions of a video encodingapparatus and a video decoding apparatus respectively performingencoding and decoding of an intra-prediction mode according to anembodiment of the present invention.

Referring to an upper portion in FIG. 12, a codeNum mapping module 1200and a codeword mapping module 1220 may be included in a video encodingapparatus. The codeNum mapping module 1200 of the video encodingapparatus may receive an intra-prediction mode number and output acodeNum. The codeNum mapping and codeword mapping according to theembodiments as described above may be performed, and a more frequentlygenerated intra-prediction mode number may be mapped to a smallercodeNum number. As described above, in case of using a method ofperforming codeNum mapping excluding an MPM, the codeNum mapping modulemay perform codeNum mapping on remaining intra-prediction modesexcluding an intra-prediction mode corresponding to the MPM.

Referring to a lower portion in FIG. 12, a video decoding apparatus mayinclude a codeword mapping module 1240 and a codeNum mapping module1260. The codeword mapping module 1240 may output a codeNum based on aninput codeword, and the codeNum mapping module 1260 may receive thecodeNum and output an intra-prediction mode number. The codeword mappingmodule 1240 may include a codeword mapping table for performing codewordmapping, and the codeNum mapping module 1260 may include a codeNummapping table for outputting an intra-prediction mode number based on areceived codeNum. The codeword mapping table and the codeNum mappingmodule may be stored in advance or adaptively generated.

What is claimed is:
 1. A decoding method of an intra-prediction mode,the method comprising: decoding flag information indicating whether oneof a candidate intra-prediction modes with respect to a current blockand an intra-prediction mode of the current block are identical; whenone of the candidate intra-prediction modes with respect to the currentblock and an intra-prediction mode of the current block are notidentical, deriving the intra-prediction mode of the current block bydecoding a syntax element of remaining intra-prediction mode informationcomprising information regarding the intra-prediction mode of thecurrent block; and generating a prediction block of the current blockbased on the derived intra-prediction mode of the current block; andgenerating a reconstructed block of the current block based on theprediction block and a residual block of the current block, wherein, thederiving the intra-prediction mode of the current block is performedbased on a table comprising intra-prediction mode information, the tablemapping intra-prediction modes and index information of theintra-prediction modes, wherein when an intra-prediction mode is aplanar mode, the intra-prediction mode is mapped to an index 0, when anintra-prediction mode is a DC mode, the intra-prediction mode is mappedto an index 1, and when intra-prediction modes are directionalintra-prediction modes, the intra-prediction modes are mapped to indices2 to 34 according to directivity of the intra-prediction modes, whereina codeword length of an index of the candidate intra-prediction modeswith respect to the current block is shorter than a codeword length ofan index of the remaining intra-prediction mode information, wherein thecandidate intra-prediction modes with respect to the current block aremodes for predicting luminance information of the current block, whereinchrominance information of the current block is predicted based onintra-prediction mode information according to the predicting luminanceinformation of the current block, and wherein the syntax element of theremaining intra-prediction mode information is a value coded using fixed5 bits.
 2. The method of claim 1, wherein a 5-bit information indicatesone of the remaining intra-prediction mode information excluding thecandidate intra-prediction modes with respect to the current block. 3.The method of claim 2, wherein the candidate intra-prediction modes withrespect to the current block comprise three different intra-predictionmodes derived based on at least one of intra-prediction modescorresponding to neighbor blocks of the current block and an additionalintra-prediction mode.
 4. The method of claim 1, wherein when the flaginformation is 1, the flag information indicates that one of thecandidate intra-prediction modes of the current block and theintra-prediction mode of the current block are identical, and when theflag information is 0, the flag information indicates that the candidateintra-prediction modes of the current block and the intra-predictionmode of the current block are not identical.
 5. The method of claim 1,wherein the decoding of the syntax element of the remainingintra-prediction mode information comprises: deriving remaining 32intra-prediction mode information, excluding 3 intra-prediction modes ascandidate intra-prediction modes with respect to the current block, inthe table; mapping the syntax element of the remaining intra-predictionmode information to the remaining 32 intra-prediction mode information;and setting one intra-prediction mode, among the remaining 32intra-prediction mode information mapped to the syntax element, as anintra-prediction mode of the current block.
 6. An encoding method of anintra-prediction mode, the method comprising: determining and encodingflag information indicating whether one of a candidate intra-predictionmodes with respect to a current block and an intra-prediction mode ofthe current block are identical; when one of a plurality of candidateintra-prediction modes with respect to a current block and anintra-prediction mode of the current block are not identical, encoding asyntax element of remaining intra-prediction mode of the current blockincluding information regarding the intra-prediction mode of the currentblock; generating a prediction block of the current block based on theintra-prediction mode of the current block; and generating areconstructed block of the current block based on the prediction blockand a residual block of the current block, wherein, the deriving theintra-prediction mode of the current block is performed based on a tablecomprising intra-prediction mode information, the table mappingintra-prediction modes and index information of the intra-predictionmodes, wherein when an intra-prediction mode is a planar mode, theintra-prediction mode is mapped to an index 0, when an intra-predictionmode is a DC mode, the intra-prediction mode is mapped to an index 1,and when intra-prediction modes are directional intra-prediction modes,the intra-prediction modes are mapped to indices 2 to 34 according todirectivity of the intra-prediction modes, wherein a codeword length ofan index of the candidate intra-prediction modes with respect to thecurrent block is shorter than a codeword length of an index of theremaining intra-prediction mode information, wherein the candidateintra-prediction modes with respect to the current block are modes forpredicting luminance information of the current block, whereinchrominance information of the current block is predicted based onintra-prediction mode information according to the predicting luminanceinformation of the current block, and wherein the syntax element of theremaining intra-prediction mode information is a value coded using fixed5 bits.